(1) Field of the Invention
This invention relates generally to a miniature motor with a worm reduction gear, and more particularly to a miniature motor with a worm reduction gear generating motor output via a worm reduction gear, in which the motor shaft of the miniature motor is extended to a predetermined length, a worm on the worm reduction gear has a shaft hole into which the motor shaft is inserted, and the worm is fixedly fitted to the motor shaft by press-fitting the motor shaft to the shaft hole of the worm, thereby making it possible to reduce the diameter of the motor shaft and the size and weight of the miniature motor with a worm reduction gear.
(2) Description of the Prior Art
A miniature motor with a worm reduction gear in which a worm is formed on an end of the motor shaft and brought in mesh with a worm wheel is known for use in drive units for automotive electrical parts, for example.
FIG. 1 shows one example of the conventional type miniature motor with a worm reduction gear. In the conventional type miniature motor with a worm reduction gear shown in the figure, a worm is formed on an extended part of a motor shaft for the sake of convenience in manufacture and assembly. In FIG. 1 illustrating the cross-section of the conventional type miniature motor with a worm reduction gear, numeral 1 refers to a motor portion; 2 to a reduction gear portion; 3 to a rotor; 4 to a motor shaft; 6 to a worm portion; 7 to a worm wheel; 8 to an output shaft; 9 to a steel ball; 10 to a thrust-bearing plate; 11 to a magnet; 12 to a commutator; 13 to brush; 14 and 15 to bearings; 16 to a motor case; and 17 to a gear case, respectively. Symbol A refers to a thrust-bearing screw; and B to a nut;
In a miniature motor with a worm reduction gear, the outside diameter of the worm is usually set to a predetermined value in accordance with the desired output and reduction gear ratio. In the case of the example shown in FIG. 1, since the worm portion 6 is formed on an extended part of the motor shaft 4, and the bearing 14 is provided on the gear case 17, the inside diameter of the bearing 14 has to be made larger than at least the outside diameter of the worm portion from considerations in assembly. As a result, the diameter of the motor shaft 4 has to be made larger than at least the outside diameter of the worm portion 6, and the outside diameter of the rotor 3 also has to be increased by that amount. This tends to make it difficult to reduce the size and weight of the motor portion, posing an unwanted problem of the difficulty in reducing cost.